In motor vehicles, there has been increasing use of automatic stepped transmissions with an automatic friction clutch as a startup and shift clutch, in which the gear selection, the triggering of shift operations, the engaging and disengaging of gear steps and the engaging and disengaging of the friction clutch are automated; that is, these operations occur by evaluating operating parameters in a transmission control device and the drive assigned to the control.
Particularly in the case of commercial vehicles, the drive engines are usually designed as turbo-charged diesel engines, which have a specific load build-up characteristic. The document DE 10 2008 054 802.2, which was not pre-published, discloses a method for controlling an automatic stepped transmission depending on the dynamic operating characteristics of a turbo-charged internal combustion engine, and describes in detail that a turbo-charged internal combustion engine can spontaneously, that is with high torque gradients, only reach an intake torque lying below the full load torque. A further increase of the engine torque is briefly possible, although with low torque gradients, only above a boost threshold speed, after which the turbo-charger creates a significant increase of the charge pressure and thus the engine torque.
Thus aside from the idle speed, cut-off speed and the full load torque characteristic curve, the dynamic behavior of a turbo-charged internal combustion engine is also determined by the boost threshold speed and the intake torque characteristic curve as well as by the present torque gradients, at least in certain regions. For turbo-charged internal combustion engines below the boost threshold speed, due to the limitation of the spontaneously achievable engine torque to the intake torque, significant torque deficiency that is generally referred to as turbo lag, is observed, which can occur for example during load build-up at the end of a tractive upshift, when the coupling speed of the target gear lies below the boost threshold speed of the drive engine.
Such torque deficiency of the drive engine occurs during a tractive upshift, particularly at high drive resistance and high engine loads, e.g., during uphill travel on steep road inclines, with high loads or on difficult terrain (off-road travel). Under these operating conditions, the coupling speed of the target gear, determined by the transmission ratio step of the upshift, can be reduced so far that the coupling speed lies below the boost threshold speed of the drive engine. Consequently, the drive engine during load build-up at the end of the tractive upshift can then spontaneously generate and produce only the intake torque thereof which can however lie below the actual drive resistance (rolling resistance+incline resistance+wind resistance). In this case, it is therefore no longer possible to continue travel at a constant travel speed or with vehicle acceleration corresponding to the gas pedal position.
It is then typically necessary to downshift or even to startup from a standstill in order to avoid lowering the speed of the drive engine below the idle speed and subsequently stalling the drive engine. However, this significantly disrupts the desired travel activity, and from the perspective of the driver is considered mostly inconvenient.
The shift strategy of an automatic stepped transmission, that is the shift characteristic lines and shift characteristics fields of the transmission control, is typically designed such that under certain operating conditions, tractive upshift does not occur for automatically triggered shifting, and is not possible for manually triggered shifting. It is possible with a tractive upshift, however, that such an operating situation with torque deficiency of the drive engine arises due a disruption.
Thus, the tractive-free phase of shifting can be quite prolonged due to problems internal to the transmission, because with the use of synchronized clutches, for instance, the synchronization of the target gear is delayed due to strong wear of the assigned friction synchronization, or with the use of unsynchronized clutches, the engagement of the target gear is delayed due to a tooth-on-tooth position of the clutch halves. Likewise, the drive resistance can increase significantly during the tractive-free phase of the tractive upshift, for example, due to transitioning to a roadway section with a significantly higher incline. Consequently, the travel speed of the vehicle decreases more strongly during the shift-dependent interruption of the tractive force than provided for in shifting characteristics lines and shifting characteristic fields of the transmission control.
It is also possible however, that such an operating state is caused intentionally with certain emergency vehicles or with certain emergency travel, in that an atypical tractive upshift starting from high engine speed is performed with an unusually large gear increment, that is, with an increased transmission ratio step. This way, acceleration to an intended target speed in the respective target gear is intended to be performed, that is without further tractive upshifts and the interruptions in tractive force associated with the upshifts. This relates to airport fire engines, for example, which must reach their respective site of action as quickly as possible, thus with the greatest possible acceleration. With the use of a turbo-charged internal combustion engine, the greatest possible gear increment that can be used for this purpose is limited however, by the load build-up characteristic of the drive engine because reaching the emergency site would be greatly delayed with the occurrence of temporary torque deficiency of the drive engine.
Numerous devices and methods for the use thereof have been disclosed for avoiding, or at least reducing, the undesired turbo lag. Thus, for example, an adjustable turbine geometry for improving the response behavior of the exhaust gas turbo-charger, a boost drive for supporting the internal combustion engine at high engine load, or additional devices for increasing the charging pressure at low engine speeds, such as a mechanically driven compressor, an electrically driven booster compressor, a mechanical or electrical drive of the drive shaft of the exhaust gas turbo-charger, or a compressed air supply device for generating and feeding compressed air into the intake and exhaust gas tract of the internal combustion engine.
Thus the document U.S. Pat. No. 6,692,406 B2 describes a gearshift control method for a motor vehicle provided with a turbo-charged internal combustion engine, an automatic friction clutch and an automatic transmission, according to which, with a full load upshift, the internal combustion engine is controlled such that the charge pressure during the shift procedure is maintained either by an increase of the exhaust gas energy or by maintaining the rotational speed of the exhaust gas turbo-charger, whereby sufficiently high engine torque can be built up at the load build-up at the end of the tractive upshift. For this purpose, among others, there can be a suitable adjustment of the turbo blades of a turbo-charger with variable turbine geometry (VTG).
A method for operating a drive train is known from the document DE 103 35 259 A1 in which, in particular, a turbo-charged internal combustion engine is driven during startup and shift operations in order to compensate the torque deficiency thereof by an electrical boost drive, whereby the engine speed of the internal combustion engine is increased more quickly in each case, and the slipping phase of the friction clutch is shorter.
Finally, the document DE 10 2006 027 865 A1 discloses a method for regulating charge pressure of a turbo-charged internal combustion engine, according to which, with a load build-up during a tractive upshift, compressed air from a compressed air supply device before the turbine of the exhaust gas turbo-charger is fed into the exhaust gas tract of the internal combustion engine, whereby the compressor of the exhaust gas turbo-charger is strongly accelerated by the turbine and thus generates an increased charge pressure.
Such additional devices are, however, relatively complex and expensive, increase the construction space requirements and represent increased failure potential for the operation of the internal combustion engine, so that they are frequently omitted.